In electro-mechanical cables the load-carrying elements are usually of steel, often high tensile, whereas the electrical transmitting elements comprise relatively ductile non-ferrous (e.g. copper) conductors and non-metallic materials (e.g. elastomers and plastics) which act as insulators. In the conventional types of electro-mechanical cables the load-carrying and the transmitting elements are spun together, typically in two distinct arrangements:
(1) transmitting elements are spun around a central load-carrying strength member, or PA0 (2) load-carrying elements are spun around a central transmitting core, producing an armoured cable.
In the former case the transmitting elements are directly exposed to stress when used in a dynamic situation. In the latter case, the load carrying armour tends to exert substantial compressive stresses on the central transmitting core. These are a direct consequence of the helical formation of the load-carrying elements (wires or strands) and the tensile load applied to the armour. This tensile load results in radial stresses which are transmitted directly to the core. The uniform radial stresses due to tensile loading will be increased and rendered non-uniform where transverse loads are applied to the cable, e.g. at dies and over sheaves or fairleads.
In the conventional armoured cable these stresses occur inevitably both during the armouring process and in subsequent use of the cable. It is well known that such stresses can cause substantial deterioration of the electrical properties (e.g. attenuation) of the core. In dynamic situations, e.g. where the cable is repeatedly reeled under load, this deterioration may continue unabated until electrical failure (e.g. open circuit or insulation breakdown) occurs.
Recently, optical fibers have been introduced into armoured strain cables for signal-carrying purposes, and these are even more sensitive to mechanical stress and deformation than copper conductors.
There have previously been proposals for cable in the form of armoured tubes. In such a case, the conductor or other transmitting element has to be threaded along the tube, which severely limits the practical length of the cable; it would be impossible to produce strain cables of the order of a kilometer in length in this way. Furthermore, the tubes occupy an excessive proportion of the cross-section of the cable or do not contribute to its load-carrying capacity. U.K. patent specification No. 325 087 discloses a stranded wire rope having a core consisting of a single metal strip coiled helically to form a flexible tube. Such a tube has a very low longitudinal load-bearing capacity and cannot be made in unlimited length.